System for drilling a wellbore

ABSTRACT

A system is disclosed for drilling a wellbore ( 1 ) into an earth formation. The system comprises an expandable tubular element ( 4 ) extending into the wellbore, whereby a lower end portion ( 11 ) of the wall of the tubular element ( 8 ) extends radially outward and in axially reverse direction so as to define an expanded tubular section ( 10 ) extending around a remaining tubular section ( 4 ) of the tubular element. The expanded tubular section is extendable by downward movement of the remaining tubular section relative to the expanded tubular section whereby said lower end portion ( 14 ) of the wall bends radially outward and in axially reverse direction.

The present invention relates to a system for drilling a wellbore intoan earth formation whereby an expandable tubular element extends intothe wellbore.

The technology of radially expanding tubular elements in wellbores findsincreasing application in the industry of oil and gas production fromsubterranean formations. Wellbores are generally provided with one ormore casings or liners to provide stability to the wellbore wall, and/orto provide zonal isolation between different earth formation layers. Theterms “casing” and “liner” refer to tubular elements for supporting andstabilising the wellbore wall, whereby it is generally understood that acasing extends from surface into the wellbore and that a liner extendsfrom a certain depth further into the wellbore. However, in the presentcontext, the terms “casing” and “liner” are used interchangeably andwithout such intended distinction.

In conventional wellbore construction, several casings are set atdifferent depth intervals, and in a nested arrangement, whereby eachsubsequent casing is lowered through the previous casing and thereforehas a smaller diameter than the previous casing. As a result, thecross-sectional wellbore size that is available for oil and gasproduction, decreases with depth. To alleviate this drawback, it hasbecome general practice to radially expand one or more tubular elementsat the desired depth in the wellbore, for example to form an expandedcasing, expanded liner, or a clad against an existing casing or liner.Also, it has been proposed to radially expand each subsequent casing tosubstantially the same diameter as the previous casing to form amonobore wellbore. It is thus achieved that the available diameter ofthe wellbore remains substantially constant along (a portion of) itsdepth as opposed to the conventional nested arrangement.

EP 1438483 B1 discloses a system for expanding a tubular element in awellbore whereby the tubular element, in unexpanded state, is initiallyattached to a drill string during drilling of a new wellbore section.

To expand such wellbore tubular element, generally a conical expander isused with a largest outer diameter substantially equal to the requiredtubular diameter after expansion. The expander is pumped, pushed orpulled through the tubular element. Such method can lead to highfriction forces between the expander and the tubular element. Also,there is a risk that the expander becomes stuck in the tubular element.

EP 0044706 A2 discloses a flexible tube of woven material or cloth thatis expanded in a wellbore by eversion to separate drilling fluid pumpedinto the wellbore from slurry cuttings flowing towards the surface.

However there is a need for an improved system for drilling a wellborewhereby an expandable tubular element extends into the wellbore.

In accordance with the invention there is provided a system for drillinga wellbore into an earth formation, comprising

an expandable tubular element extending into the wellbore, whereby alower end portion of the wall of the tubular element extends radiallyoutward and in axially reverse direction so as to define an expandedtubular section extending around a remaining tubular section of thetubular element, the expanded tubular section being extendable bydownward movement of the remaining tubular section relative to theexpanded tubular section whereby said lower end portion of the wallbends radially outward and in axially reverse direction; and

a drill string extending through the remaining tubular section, whereinthe tubular element and the drill string are arranged for transferring athrust force from the remaining tubular section to the drill string, andwherein the drill string includes a jetting head for deepening thewellbore by jetting a stream of fluid against the bottom of thewellbore.

By moving the remaining tubular section downward relative to theexpanded tubular section during drilling, the tubular element iseffectively turned inside out whereby the lower end portion of the wallof the tubular element is continuously bent radially outward and inaxially reverse direction so that the tubular element is progressivelyexpanded without the need for an expander that is pushed, pulled orpumped through the tubular element. In this manner the expanded tubularsection forms a casing or liner that is installed in the wellbore duringthe drilling process, so that a relatively short open-hole section canbe maintained during drilling.

Furthermore, the thrust force transmitted from the remaining tubularsection to the drill string can be kept small since the jetting headrequires only a small thrust force when compared to the large thrustforce required for drilling with a conventional drill bit. In viewthereof, the downward force that must be exerted to the remainingtubular section to move it downward includes only a small component forthrusting the drill string during drilling. It is thereby achieved thatthe risk of exceeding the yield strength of the remaining tubularsection is significantly reduced, when compared to drilling with aconventional drill bit.

Suitably the system of the invention comprises means for centralisingthe jetting head in the remaining tubular section.

In a preferred embodiment, the drill string is provided with a reamerfor reaming the wellbore to at least an outer diameter of the expandedtubular section.

To maintain a short open-hole section while drilling, it is preferredthat the remaining tubular section and the drill string are arranged forsimultaneous lowering in the wellbore whereby, for example, said lowerend portion of the wall is arranged for lowering into the wellbore atsubstantially the same speed as the speed of lowering of the drillstring during drilling of the wellbore.

It is preferred that the wall of the tubular element includes a materialthat is plastically deformed during the bending process, so that theexpanded tubular section retains an expanded shape as a result of saidplastic deformation. Thus, there is no need for an external force orpressure to be applied to the expanded tubular section to maintain itsexpanded form. If, for example, the expanded tubular section is expandedagainst the wellbore wall as a result of said bending of the wall, noexternal radial force or pressure needs to be exerted to the expandedtubular section to keep it against the wellbore wall. Suitably the wallof the tubular element is made of a metal such as steel or any otherductile metal capable of being plastically deformed by eversion of thetubular element. The expanded tubular section then has adequate collapseresistance, for example in the order of 100-150 bars.

Suitably the remaining tubular section is induced to move downward whilethe expanded tubular section is kept stationary in the wellbore.

In order to induce said downward movement it is preferred that theremaining tubular section is subjected to an axially compressive force,which at least partly can result from the weight of the remainingtubular section. If necessary the weight can be supplemented by anexternal, downward, force applied to the remaining tubular section toinduce said movement. As the length, and hence the weight, of theremaining tubular section increases, an upward force may need to beapplied to the remaining tubular section to prevent uncontrolled bendingor buckling of the wall.

Suitably the remaining tubular section is axially extended at its upperend in correspondence with its downward movement. This is done, forexample, by adding tubular portions at the upper end in any suitablemanner such as by welding. Alternatively, the remaining tubular sectionis formed as a coiled tubing that is unreeled from a reel andsubsequently inserted into the wellbore. In this way the process ofeversion of the tubular element can be continued until a desired lengthof the tubular element is expanded.

The invention will be described hereinafter in more detail and by way ofexample, with reference to the accompanying drawing in which:

FIG. 1 schematically shows a lower portion of a first embodiment of thesystem of the invention;

FIG. 2 schematically shows the first embodiment during cutting of atubular element in a wellbore; and

FIG. 3 schematically shows a lower portion of a second embodiment of thesystem of the invention.

In the drawing and the description, like reference numerals relate tolike components.

Referring to FIGS. 1 and 2, there is shown a system including a wellbore1 extending into an earth formation 2, and an expandable tubular elementin the form of liner 4 extending from surface downwardly into thewellbore 1. The liner 4 has been partially radially expanded by eversionof the wall of the liner whereby a radially expanded tubular section 10of the liner 4 has been formed, which has an outer diametersubstantially equal to the wellbore diameter. A remaining tubularsection of the liner 4, in the form of unexpanded liner section 8,extends concentrically within the expanded tubular section 10.

The wall of the liner 4 is, due to eversion at its lower end, bentradially outward and in axially reverse (i.e. upward) direction so as toform a U-shaped lower section 11 of the liner interconnecting theunexpanded liner section 8 and the expanded liner section 10. TheU-shaped lower section 11 of the liner 4 defines a bending zone 12 ofthe liner.

The expanded liner section 10 is axially fixed to the wellbore wall 14by any suitable anchoring means (not shown), or by frictional forcesbetween the expanded liner section 10 and the wellbore wall 14 resultingfrom the expansion process. The U-shaped lower section 11 of liner 4 ispositioned a short distance above the bottom of the wellbore so that anopen-hole wellbore section 13 is defined below the U-shaped lowersection 11.

A drill string 20 extends from surface through the unexpanded linersection 8 to the bottom of the wellbore 1, with a jetting head 22 at itslower end. The jetting head 22 comprises a plurality of jetting nozzles24 and cutting nozzles 26. The jetting nozzles 24 are directed so as toeject fluid jets 28 against the bottom and/or the wall of the wellbore1. Each fluid jet 28 suitably includes a stream of fluid, e.g. water,with abrasive particles entrained therein. The cutting nozzles 26 aredirected radially outward from the jetting head.

The drill string 20 is provided with a guide device 30 having a curvedsurface portion 32 arranged to transfer a thrust force from the liner 4to the drill string 20, and to support and guide the U-shaped lowersection 11 of the liner 4 during eversion of the liner 4. The guidedevice 30 is rotatable relative to the drill string 20 about its centrallongitudinal axis. Furthermore, the guide device 30 is collapsible so asto allow it to pass through the unexpanded liner section 4 (FIG. 2).

In FIG. 3 is shown the second embodiment, which is substantially similarto the first embodiment, except that the cutting head 22 is providedwith reamers 33 adapted to ream the wellbore 1 to a nominal diametersubstantially equal to the outer diameter of the expanded liner section10. The reamers 33 are radially retractable to allow the reamers 33 topass through the unexpanded liner section 4 when in retracted mode.

During normal operation or the first embodiment (FIGS. 1 and 2), a lowerend portion of the liner 4 is initially everted. That is, the lower endportion is bent radially outward and in axially reverse direction,whereby the U-shaped lower section 11 and a short length of expandedliner section 10 are initiated. Subsequently, the expanded liner section10 is anchored to the wellbore wall 14 by the anchoring means. Dependingon the geometry and/or material properties of the liner 4, the expandedliner section 10 alternatively can become anchored to the wellbore wallautomatically by friction forces between the expanded liner section 10and the wellbore wall 14.

A downward force is then applied to the unexpanded liner section 8 so asto move it gradually downward. As a result, the unexpanded liner section8 becomes progressively everted whereby the unexpanded liner section 8is transformed into the expanded liner section 10. The bending zone 12moves in downward direction during the eversion process at approximatelyhalf the speed of downward movement of the unexpanded liner section 8.

If desired, the mechanical properties and dimensions of the liner 4 canbe selected such that the expanded liner section 10 becomes pressedagainst the wellbore wall 14 as a result of the expansion process so asto seal against the wellbore wall and/or to stabilize the wellbore wall.

Since the length, and hence the weight, of the unexpanded liner section8 gradually increases, the magnitude of the downward force can begradually lowered in correspondence with the increasing weight ofunexpanded liner section 8. Eventually, the downward force may need tobe replaced by an upward force to prevent buckling of liner section 8.

The unexpanded liner section 8 is at its upper end extended incorrespondence with its downward movement, for example by adding tubularsections to the liner, or by continuously forming the liner from metalsheet on a reel.

Simultaneously with downward movement of the unexpanded liner section 8into the wellbore, the drill string 20 is operated to deepen thewellbore bottom by ejecting the fluid jets 28 against the wellborebottom whereby the drill string is rotated slowly. The drill string 20thereby gradually moves downward into the wellbore 1. The force appliedto the unexpanded liner section 4 is controlled such that the U-shapedsection 11 of the liner 4 moves downward at the same speed as the drillstring 20 and remains in contact with the curved surface portion 32 ofguide device 30 whereby the U-shaped lower section 11 exerts a smallthrust force to the drill string 20. With increasing eversion of theliner 4, an increasing portion of the thrust force results from theweight of the unexpanded liner section 8.

Since the jetting head 22 requires only a small thrust force forexcavating the wellbore, relative to the thrust force required fordrilling with a conventional drill bit, the compressive load inunexpanded liner section 4 can be kept relatively low. It is therebyachieved that the risk of inadvertent buckling of the unexpanded linersection 4 is significantly reduced.

If it is required to cut the unexpanded liner section 4, the guidedevice 30 is collapsed and the drill string 20 is raised until thecutting nozzles 26 are positioned at the desired cutting level (FIG. 2).Subsequently fluid jets 36 with entrained abrasive particles are jettedthrough cutting nozzles 26 against the unexpanded liner section 4thereby cutting the liner section 4.

Normal operation of the second embodiment is substantially similar tonormal operation of the first embodiment. In addition, the reamers 33are kept in expanded mode during drilling with the drill string 20thereby enlarging the diameter of the wellbore 1 to the nominaldiameter.

In a modified version of the second embodiment (not shown), the drillstring is at its lower end provided with a conventional pilot drill bitfor drilling a pilot bore of relatively small diameter, and the reamersare provided with jetting nozzles to enlarge the borehole to its nominaldiameter. Since the jetting nozzles can be precisely controlled withrespect to direction and velocity of the fluid jets, the jetting nozzlesin the reamers allow accurate drilling of the borehole to its nominaldiameter.

When it is required to retrieve the drill string 20 to surface the guidedevice 30 and the reamers 33 (if present) are radially retracted and thedrill string 20 is retrieved through the unexpanded liner section 8.

With the system of the invention, it is achieved that the wellbore isprogressively lined with the everted liner directly above the jettinghead during the drilling process. As a result, there is only arelatively short open-hole section of the wellbore at all times. Theadvantages of such short open-hole section will be most pronouncedduring drilling into a hydrocarbon fluid containing layer of the earthformation. In view thereof, for many applications it will be sufficientif the process of liner eversion during drilling is applied only duringdrilling into the hydrocarbon fluid reservoir, while other sections ofthe wellbore are lined or cased in conventional manner. Alternatively,the process of liner eversion during drilling may be commenced atsurface or at a selected downhole location, depending on circumstances.

In view of the short open-hole section during drilling, there is asignificantly reduced risk that the wellbore fluid pressure gradientexceeds the fracture gradient of the rock formation, or that thewellbore fluid pressure gradient drops below the pore pressure gradientof the rock formation. Therefore, considerably longer intervals can bedrilled at a single nominal diameter than in a conventional drillingpractice whereby casings of stepwise decreasing diameter must be set atselected intervals.

Also, if the wellbore is drilled through a shale layer, such shortopen-hole section eliminates possible problems due to a heaving tendencyof the shale.

After the wellbore has been drilled to the desired depth and the drillstring has been removed from the wellbore, the length of unexpandedliner section that is still present in the wellbore can be left in thewellbore or it can be cut-off from the expanded liner section andretrieved to surface.

In case the length of unexpanded liner section is left in the wellbore,there are several options for completing the wellbore. These are, forexample, as follows:

-   A) A fluid, for example brine, is pumped into the annulus between    the unexpanded and expanded liner sections so as to pressurise the    annulus and increase the collapse resistance of the expanded liner    section. Optionally one or more holes are provided in the U-shaped    lower section to allow the pumped fluid to be circulated.-   B) A heavy fluid is pumped into the annulus so as to support the    expanded liner section and increase its collapse resistance.-   C) Cement is pumped into the annulus in order to create, after    hardening of the cement, a solid body between the unexpanded liner    section and the expanded liner section, whereby the cement may    expand upon hardening.-   D) The unexpanded liner section is radially expanded (i.e. clad)    against the expanded liner section, for example by pumping, pushing    or pulling an expander through the unexpanded liner section.

In the above examples, expansion of the liner is started at surface orat a downhole location. In case of an offshore wellbore whereby anoffshore platform is positioned above the wellbore, at the watersurface, it can be advantageous to start the expansion process at theoffshore platform. In such process, the bending zone moves from theoffshore platform to the seabed and from there further into thewellbore. Thus, the resulting expanded tubular element not only forms aliner in the wellbore, but also a riser extending from the offshoreplatform to the seabed. The need for a separate riser from is therebyobviated.

Furthermore, conduits such as electric wires or optical fibres forcommunication with downhole equipment can be extended in the annulusbetween the expanded and unexpanded sections. Such conduits can beattached to the outer surface of the tubular element before expansionthereof. Also, the expanded and unexpanded liner sections can be used aselectricity conductors to transfer data and/or power downhole.

Since any length of unexpanded liner section that is still present inthe wellbore after completion of the eversion process, will be subjectedto less stringent loading conditions than the expanded liner section,such length of unexpanded liner section may have a smaller wallthickness, or may be of lower quality or steel grade, than the expandedliner section. For example, it may be made of a material having arelatively low yield strength or relatively low collapse rating.

Instead of leaving a length of unexpanded liner section in the wellboreafter the expansion process, the entire liner can be expanded with themethod described above so that no unexpanded liner section remains inthe wellbore. In such case, an elongate member, for example a pipestring, can be used to exert the necessary downward force to theunexpanded liner section during the last phase of the eversion process.

In order to reduce friction forces between the unexpanded and expandedliner sections during the expansion process, suitably a frictionreducing layer, such as a Teflon layer, is applied between theunexpanded and expanded liner sections. For example, a friction reducingcoating can be applied to the outer surface of the liner beforeexpansion. Such layer of friction reducing material furthermore reducesthe annular clearance between the unexpanded and expanded sections,which results in a reduced tendency of the unexpanded section to buckle.Instead of, or in addition to, such friction reducing layer,centralizing pads and/or rollers can be applied between the unexpandedand expanded sections to reduce the friction forces and the annularclearance there-between.

Instead of expanding the expanded liner section against the wellborewall (as described), the expanded liner section can be expanded againstthe inner surface of another tubular element already present in thewellbore.

1. A system for drilling a wellbore into an earth formation, comprisingan expandable tubular element extending into the wellbore, wherein alower end portion of the wall of the tubular element extends radiallyoutward and in an axially reverse direction so as to define an expandedtubular section extending around a remaining tubular section of thetubular element, the expanded tubular section being extendable bydownward movement of the remaining tubular section relative to theexpanded tubular section such that said lower end portion of the wallbends radially outward and in an axially reverse direction; and a drillstring extending through the remaining tubular section, wherein thetubular element and the drill string are arranged for transferring athrust force from the tubular element to the drill string, and whereinthe drill string includes a jetting head for deepening the wellbore byjetting a stream of fluid against the bottom of the wellbore.
 2. Thesystem of claim 1, further comprising means for centralising the jettinghead in the remaining tubular section.
 3. The system of claim 1 whereinthe expanded tubular section has an outer diameter, and wherein thedrill string is provided with a reamer for reaming the wellbore to atleast the outer diameter of the expanded tubular section.
 4. The systemof claim 1 wherein the remaining tubular section and the drill stringare arranged for simultaneous lowering in the wellbore.
 5. The system ofclaim 4, wherein said lower end portion of the wall is arranged forlowering into the wellbore at substantially the same speed as loweringof the drill string during drilling of the wellbore.
 6. The system ofclaim 1 wherein the wall of the tubular element includes a materialsusceptible of plastic deformation during said bending of the wall sothat the expanded tubular section retains an expanded shape as a resultof said plastic deformation.
 7. The system of claim 1 wherein theremaining tubular section is subjected to an axially compressive forceacting to induce said downward movement of the remaining tubularsection.
 8. The system of claim 7, wherein said axially compressiveforce results at least partly from the weight of the remaining tubularsection.
 9. The system substantially as described hereinbefore withreference to the drawings.